Menachot 37a מנחות ל״ז א

Rabbi Yosei HaḤorem says: This is no proof, as we have found that the right hand is also called yad, as it is stated: “And when Joseph saw that his father was laying his right hand [yad yemino]” (Genesis 48:17). The Gemara asks: And the other tanna, who maintains that the right hand is not called yad, how does he respond to this proof? He maintains that the right hand is called “his right hand [yad yemino],” but it is not called a yad without further specification.

Rabbi Natan says: This proof is not necessary, as it says: “And you shall bind them for a sign upon your arm” (Deuteronomy 6:8), and then it states: “And you shall write them upon the doorposts of your house” (Deuteronomy 6:9). This teaches that just as writing is with the right hand, as most people write with their right hands, so too, the binding of phylacteries must be performed with the right hand. And since binding is with the right hand, this means that donning is on the left arm, as one cannot bind the phylacteries with the same hand upon which he is donning them. The Gemara asks: And from where does Rabbi Yosei HaḤorem, who holds that the right hand is also called yad in the Torah, derive that donning phylacteries is on the left arm? The Gemara answers: He derives it from where Rabbi Natan derives it.

Rav Ashi said: The requirement that phylacteries be donned on the left arm is derived from the verse: “It shall be for a sign upon your arm [yadkha]” (Exodus 13:16), which is written with a letter heh at the end. This is expounded as though it stated: Your weak [keha] arm. Rabbi Abba said to Rav Ashi: But one can say that yadkha should be interpreted as yadko’aḥ, with a letter ḥet at the end instead of a heh. If so, this would mean: Your arm that is of strength [shebeko’aḥ], which is the right arm. Rav Ashi said to Rabbi Abba: Is this word written with a ḥet?

The Gemara notes that Rav Ashi’s opinion, that the halakha that phylacteries are donned on the left arm is derived from the term yadkha, is subject to a dispute between tanna’im, as it is taught in a baraita: Yadkha is written with a heh, indicating weakness, and this is referring to the left arm. Others say: “Your arm,” i.e., yadkha, serves to include one without a complete arm, i.e., one whose arm ends at the elbow, in the obligation to don phylacteries, as the remaining part is also categorized as a weak arm. It is taught in another baraita: If one does not have a left arm, i.e., not even above the elbow, he is exempt from the mitzva of phylacteries. Others say: Yadkha serves to include one without a left arm even above the elbow, teaching that he must don phylacteries on his right arm.

The Sages taught in a baraita: A left-handed person dons phylacteries on his right arm, which is equivalent to his left arm, i.e., his weaker arm. The Gemara raises a difficulty: But isn’t it taught in a baraita that a left-handed person dons phylacteries on his left arm, which is the left arm of every other person? Abaye said: When that baraita is taught, it is referring to one who has equal control with both his hands, i.e., an ambidextrous person. Since such an individual also uses his right hand, he dons phylacteries on his left arm.

The school of Menashe taught with regard to the verse: “And you shall bind them for a sign on your arm, and they shall be as frontlets between your eyes” (Deuteronomy 6:8): “On your arm”; this is the bicep. “Between your eyes”; this is the crown of the head. The Gemara asks: Where exactly on the crown of the head are the phylacteries placed? The school of Rabbi Yannai say: Phylacteries are placed on the place where the bone above the baby’s brain is soft after birth.

§ The Sage Peleimu raised a dilemma before Rabbi Yehuda HaNasi: In the case of one who has two heads, on which of them does he don phylacteries? Rabbi Yehuda HaNasi said to him: Either get up and exile yourself from here or accept upon yourself excommunication for asking such a ridiculous question. In the meantime, a certain man arrived and said to Rabbi Yehuda HaNasi: A firstborn child has been born to me who has two heads. How much money must I give to the priest for the redemption of the firstborn? A certain elder came and taught him: You are obligated to give him ten sela, the requisite five for each head.

The Gemara asks: Is that so? But Rami bar Ḥama teaches: Since it is stated with regard to the redemption of the firstborn: “The firstborn of man you shall redeem” (Numbers 18:15), I would derive that even if he was ravaged, e.g., by an animal, within thirty days of his birth, one should redeem him. To counter this, the verse states: